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Super-Earths in habitable zone may just be dead worlds

A new planetary formation model suggests that Super-Earths identified as being in the "habitable zone" by the Kepler telescope, may just be dead worlds.

The model, developed by Helmut Lammer and his team at the Space Research Institute of the Austrian Academy of Sciences, is based upon the principles of how we understand planet formation to happen. This dictates that gravity pulls together collapsing clouds of dust and ionised gases, including hydrogen and helium, until a hot core develops to become the system's sun. The remaining material collapses to form a protoplanetary disk. Planetisimals rotating in the disk collide and eventually begin to clump together to form the beginning rocky cores of planets. The forces exerted by those cores attract hydrogen from the remainging disk, but the star the planet rotates will work against this by destroying it with ultraviolet light. It's this later stage of formation that Lammer's model is based on. It weighs up how much hyrdogen would be gained and lost, depending on the rocky core's original mass.

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Super-Earths are of course so called for their mass, which is greater than Earth's but still less than that of gas giants like Uranus. Lammer and his team focused on those early planetary cores (or protoplanets) deemed to be in the habitable zone of their Sun-like star -- where planets can, according to their relative mass, support the right pressures needed for liquid to thrive on their surface -- and those with a similar mass to Earth (with planetary cores around 0.1 times that of Earth's) up to around five times the mass of Earth's, equivalent to a Super-Earth's.

Nasa's Kepler telescope has already identified many hundreds of new planets over the past few years, and of those in the habitable zone there are a number of Super-Earths that one recent study in

Astrobiology argues could be better human habitats than our own Earth. Coauthor on the paper Rene Heller from McMaster University explained his justification thus: "The Earth just scrapes the inner edge of the solar system's habitable zone -- the area in which temperatures allow Earth-like planets to have liquid surface water. So from this perspective, Earth is only marginally habitable. That led us to ask: could there be more hospitable environments for life on terrestrial planets?" In the study Heller and coauthor John Armstrong of Weber State University postulated that an ideal planet would feature more shallow bodies of water rather than a few big oceans, a better "thermostat" that would help prevent ice ages and a magnetic shield. They conclude rocky Super-Earths might fit the bill, and should be investigated, while other recent studies have identified a few watery Super-Earths with great potential to be habitable.

Lammer's model contradicts this theory, somewhat.

According to his model, Super-Earths retain the majority of their hydrogen because of their dense core. "Our results suggest that worlds like these two Super-Earths,

[the recently identified Kepler-62e and Kepler-62f], may have captured the equivalent of between 100 and 1,000 times the hydrogen in the Earth's oceans, but may only lose a few percent of it over their lifetime. With such thick atmospheres, the pressure on the surfaces will be huge, making it almost impossible for life to exist."

In contrast, a planet like ours has the right amount of hydrogen because its lower mass allows it to lose hydrogen when the Sun's strong ultraviolet light in the early years destroyed it.

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Like any model, though, it's based off of assumptions we have developed from our understanding of how the Solar System developed.

Another star might prove to interact dramatically differently, though Lammer's model attempts to take all likely eventualities into account. For instance, he and his coauthors write: "If a nebula has a low dust depletion factor or low accretion rates resulting in low protoplanetary luminosities, it is possible that even protoplanets with Earth-mass cores may keep their hydrogen envelopes during their whole lifetime."